1. Field of the Invention
This invention relates to a printed-wiring board including a multi-layered board, and more particularly relates to a structure of a land part of a through hole that is suitable in soldering between conducting bodies of a wiring pattern formed on both sides (front side and back side) of a board with lead-free solder.
2. Description of the Related Art
At first, the structure of a related printed-wiring board will be described with reference to the drawings.
FIG. 5 is a plan view of a component side around one through hole part of a related printed-wiring board, FIG. 6 is a plan view of the soldering side of the printed-wiring board shown in FIG. 5, FIG. 7 is a cross sectional side view of the cross sectional side view along the line Axe2x80x94A of the printed-wiring board shown in FIG. 5, and FIG. 8 is a cross sectional view for describing undesirable phenomenon that arises when the printed-wiring board shown in FIG. 5 is used for soldering.
A both-side printed-wiring board having the structure in which an electrically conducting layer is formed on the through hole surface by means of plating to electrically connect between conducting bodies on both sides (front side and back side) has been used widely for various electronic apparatuses as the low cost both-sides printed-wiring board.
As an example of the related both-sides printed-wiring board of this type, only one through hole part is shown in FIG. 5 to FIG. 8. In these drawings, the character 1 denotes the printed-wiring board. On the printed-wiring board 1, a wiring pattern formed by forming land portions 3a, 3b on both sides (front side and back side) of a board 2 consisting of insulative material such as glass epoxy resin is formed, and an electrically conducting layer 5 is formed on the inside peripheral surface of a through hole 4 formed through the board 2 from one land portion 3a to the other land portion 3b by means of plating to connect between both land portions 3a and 3b. The part of the wiring pattern other than both land portions 3a and 3b is covered with a layer 6 of insulative material (referred to as xe2x80x9cinsulating layerxe2x80x9d hereinafter) such as solder resist.
The insulating layer 6 provided on both sides (front side and back side) of the printed-wiring board 1 covers the other part of the wiring pattern so as not to cover both land portions 3a and 3b, and as a matter of course the inside diameter Db of the insulating layer 6 is larger than the diameter Da of the through hole 4.
When a lead part, which is one type of electronic parts, is soldered on the printed-wiring board 1 having the structure described hereinabove, a lead L is inserted from a component side 1A, the head end of the lead L exposed to an opposite soldering side 1B is soldered to the land part 3b by use of solder S so that a fillet Sb is formed, and solder that fills in and passes through the through hole 4 forms a fillet Sa and solders a base of the lead L to the land part 3a. 
However, in the case that lead-free solder is used for soldering, when lead-free solder is solidified, the fillet Sa on the component side 1A is partially separated from the land part 3a as shown with the arrow C or the fillet Sa lifts the land part 3a as shown with arrow D, that is, so-called lift-off phenomenon occurs.
The lift-off phenomenon does not occur when lead-containing solder is used and is special to lead-free solder, and occurs regardless of soldering process, including soldering process by use of a jet soldering tank and soldering bell. The part where lift-off phenomenon occurs does not depend on the type of soldering process. The lift-off phenomenon is likely due to the heat shrinkage or solidification shrinkage of lead-free solder itself, and a method for controlling or preventing the lift-off phenomenon has not been established yet.
It is required to prevent the lift-off phenomenon perfectly because a signal line lead out from the land part 3a is disconnected or a signal line (not shown in the drawing) is separated problematically when the land part 3a on the component side 1A is lifted, also because the long term reliability of solder junction where the lift-off phenomenon occurs is poor.
The present invention has been accomplished in view of the above-mentioned problem, and it is the object of the present invention to provide a printed-wiring board that is capable of preventing the lift-off phenomenon without changing the related fabrication process for fabricating a related printed-wiring board.
According to the invention, the above-mentioned problem is solved by applying a printed-wiring board, including a wiring pattern and land portions which are formed on a component side having electric components thereon and a soldering side of the board respectively, in which both of the land portions are connected with each other through an electrically conducting layer formed on an inside peripheral surface of a through hole bored through the board, and the wiring pattern is covered with an insulating layer, and in which one of the land portions on said component side is covered with the insulating layer.
When an electronic parts is soldered with lead-free solder, a lead-free solder fillet that has filled in and passed through the through hole from the soldering side stops at the above-mentioned component side end of the through hole, and the land part on the soldering side can secure the same soldering surface as that of the related printed-wiring board.
Therefore, electronic parts are soldered by means of a related soldering process, the lift-off phenomenon is prevented perfectly, separation of the land part and disconnection of wiring on the component side will not occur, and the reliability of a printed-wiring board is improved significantly.